A switching power supply device is used as an AC (Alternating Current)/DC (Direct Current) converter or a DC/DC converter. One conventional configuration is an isolated switching power supply device where a primary-side circuit (in an AC/DC converter, the circuit on the side that receives power from an AC power supply) and a secondary-side circuit (in an AC/DC converter, the circuit on the side that outputs a DC voltage) are magnetically coupled but electrically isolated using a transformer. In an isolated switching power supply device, electrical shorting that occurs on one of the primary or secondary sides is prevented from affecting the other side. As one example, when an overvoltage occurs in the primary-side circuit, which may be caused by a lightning strike, it is possible to protect the equipment connected to the secondary-side circuit.
One example of a circuit for rectifying a voltage waveform generated by the secondary winding of a transformer is a synchronous rectifier circuit that includes a transistor connected to the secondary winding and rectifies the voltage by turning the transistor on or off at timing based on the voltage waveform. When a rectifier circuit composed of a diode and a capacitor is used, the conversion efficiency of the converter is restricted by the large power loss caused by the on voltage of the diode and the current flowing in the diode. With a synchronous rectifier circuit however, the on voltage of the transistor is low, which suppresses the power loss.
There is also a conventional technology where an auxiliary winding is provided on the secondary side of a transformer and the transistor in a synchronous rectifier circuit is controlled based on a voltage induced in the auxiliary winding (see for example Japanese Laid-open Patent Publication No. H08-223906, Japanese Laid-open Patent Publication No. H08-331842, and Japanese Laid-open Patent Publication No. H11-136934). However, in recent years, it has become common to use a control circuit, such as a dedicated control integrated circuit (IC), to precisely control the transistor of the synchronous rectifier circuit and thereby raise the conversion efficiency.
A switching power supply device has a discontinuous current mode, a critical current mode, and a continuous current mode as operation modes. In discontinuous current mode, transistors provided for switching purposes in the primary-side circuit and in the synchronous rectifier circuit are controlled so that the currents flowing the respective transistors both become 0 A at the same time for a certain period in each cycle in the current waveform. In critical current mode, each transistor is controlled so that the currents flowing in the transistors are both 0 A at one point in each cycle of the current waveform. In continuous current mode, each transistor is controlled so that there is no period or timing where the currents flowing in the respective transistors are 0 A at the same time. In continuous current mode, a larger output current may be obtained than in the other modes.
In a switching power supply device that controls the transistor in a synchronous rectifier circuit using a control circuit like that described above, when using continuous current mode, there has been the risk of the voltage at the terminals of a transistor connected to the secondary winding reaching a large positive voltage before the transistor is turned off. When the voltage at the terminals reaches a large positive value, a large current flows via the transistor toward the reference potential, producing a large power loss. This means that with a switching power supply device that uses isolated synchronous rectification, it has been difficult to achieve a higher output using continuous current mode.